Examples on using Psychart-- The digital psychrometric software

Wei Fang, Ph.D., Professor

Dept. of Bio-Industrial Mechatronics Engineering

National Taiwan University Email: weifang@ntu.edu.tw

Properties of the moist air

1. Moist air at standard atmospheric pressure (Patm) is at 20 degree C dry-bulb temperature (Tdb) and 50 % relative humidity (RH). What is the humidity ratio (absolute humidity, AH), degree of saturation (DOS), dew point temperature (Tdp), enthalpy (h), and wet bulb temperature (Twb) of the moist air ?

Set the atmospheric pressurePatm

Properties of the moist air

2. Continuing with problem 1, what is the wet bulb depression (WBD) and what is the vapor pressure deficit (VPD) ?

WBD = Tdb – Twb = 20 – 13.79 = 6.21 deg. C

VPD = Pws – Pw = 2.339 – 1.169 = 1.17 kPa

Dry-bulb temperature (Tdb) and relative humidity (RH) in greenhouse

Tdb, deg.C

RHmin

(Activate fogging system if below this

value)

Proper RH

RHmax (severe disease problem might occurred if higher than this

value)

15 - 50 % 73

20 46 64 80

25 60 73 86

30 70 80 89

VPD is a better index

Vapor Pressure Deficit

VPD

VPD too large(activate fogging

system)

Proper VPDrange

VPD too low(good for bacterial)

(activate heating or dehumidification)

VPD too low (might cause severe disease

problem)

1.25kPa

0.85kPa

< 0.45 kPa

< 0.20kPa

T, RH and VPD in a Greenhouse located in Taiwan

Temperature, deg.C

Relative humidity, %

VPD, kPa

Properties of the moist air

3. Fill the following blanks.

#1 #2 #3 #4

Atmospheric pressure, kPa 101.325 101.325 101.325 89.874

Dry-bulb T, deg.C 20 20 -10 20

Relative humidity, % 50 80 50 50

Humidity ratio, kg/kg

Dew point T, deg.C

Wet bulb T, deg.C

Specific volume, m3/kg

A bug was found in the software.

Twb should vary according to Patm.

Psychrometric Process

4. Determine the final state of moist air originally at 20 deg.C and 50 % relative humidity

if 10 kJ are removed from 1.2 kg of the air.

Answer to Problem 4 T = q/(Ma*Cp)=(-10 kJ)/(1.2 kg*1.006 kJ/kg.K)= -8.28 K• The final dry bulb T will be 20 – 8.28 = 11.72 deg.C if this is a sensib

le heating process.• The dew point T of moist air at initial state = 9.15 deg.C

• The final dry bulb T > the dew point T of initial state, this is a sensible cooling process.

h =q/Ma = (-10 kJ)/(1.2 kg) = - 8.333 kJ/kg• The initial h = 38.5 kJ/kg• The final h = 38.545 – 8.333 = 30.212 kJ/kg• The initial dew point T = final dew point T.• Final state can be found based on final Tdb and Tdp, or

Tdb and h, or Tdp and h

State 2 was determined based on Tdb and H

H = 38.545 – 8.333 = 30.212 kJ/kg

Diff. of Tdp should be 0.

State 2 was determined based on Tdb and Tdp

H = 8.591 kJ/kg which is a little bit off from 8.33 kJ/kg, also Diff. of AH should be 0.

Psychrometric Process

5. Determine the final state of moist air originally at 20 deg.C and 50 % relative humidity

if 20 kJ are removed from 1.2 kg of the air.

Answer to Problem 5 T = q/(Ma*Cp)=(-20kJ)/(1.2kg*1.006kJ/kg.K)= -16.56 K• The final T will be 20 – 16.56 = 3.44 deg.C if this is a sen

sible heating process.• The dew point T of moist air at initial state = 9.1 deg.C• The final dry bulb T < dew point at initial state, this is a c

ooling with condensation/dehumidification process.

• The final relative humidity is 100 %.

h =q/Ma = (-20 kJ)/(1.2 kg) = - 16.66 kJ/kg• The initial h = 38.545 kJ/kg• The final h = 38.545 – 16.66 = 21.885 kJ/kg

State 2 can be determined based on RH and H,

however, this software does not provide this option. We need to use trial and error method on Tdb of state 2 to derive H = -16.66 kJ/kg..

Psychrometric Process

6. Greenhouse in tropical and subtropical regions often encounter high humidity problem. For some floral industries, dehumidifier is used in the greenhouses. Assuming the night time Tdb and RH in a 3 m height 1000 m2 greenhouse before turn on the dehumidifier is 18 degree C, 91 % respectively. After turn on the dehumidifier for a while, the Tdb remain the same and RH drop to 80 %. How much water was removed from the moist air inside this greenhouse ?

Answer to Problem 6

• Before use– Specific volume = 0.8404 m3/kg– AH = 0.0117 kg vapor/kg dry air

• After use– AH = 0.0103 kg vapor/kg dry air

• Difference in AH– AH1 – AH2 = 0.0014 kg vapor/kg dry air– Air volume 1000 x 3 = 3000 m3

– Air weight = 3000 / 0.8404 = 3569.7 kg– Total water condensed = 3569.7 * 0.0014 =

4.997 kg

Psychrometric Process

7. Air at 30 deg.C and 60% relative humidity in a growth chamber is cycled past the cooling coils and is returned back to the chamber at a temperature of 15 deg.C. Determine the psychrometric properties of the air after it is cooled, the sensible and latent heat removed, and the water vapor condensed per kg of dry air moved past the coils.

Growth Chamber

30 deg.C

60 % relative humidity

Cooler

15 deg.C

29.16 kJ/kg is the total heat removed, including the sensible heat and the latent heat.

15.378 kJ/kg is the sensible heat removed

13.782 kJ/kg is the latent heat removed

0.0054 kg/kg dry air is the amount of water removed

13.782/ 2428.4 = 0.00567 kg/kg dry air is the amount of water removed based on Hfg.

Psychrometric Process

8. When ambient conditions are 35 deg.C and 25 % relative humidity, (a). determine the dry bulb temperature to which ventilation air could be cooled if drawn through an evaporative cooler with an efficiency of 75%. (b). Calculate how much water must be added to each cubic meter of the air drawn through the cooler. (c). Redo problem 7(a) with the ambient conditions at 35 deg.C and 80 % relative humidity.

Answer to problem 8

• (a). At the given condition: • Tdb = 35 deg.C• RH = 25 % (Very dry weather)

• Answer•Twb = 20.15 deg.C• WBD = 35 – 20.15 = 14.85 deg.CT = 0.75 * WBD = 11.13 deg.C• Final T = 35 – 11.13 = 23.87 deg.C

WBD

Answer to problem 8

• (c). At the given condition: • Tdb = 35 deg.C• RH=80 % (Regular humid weather)

• Answer• Twb = 31.64 deg.C • WBD = 35 – 31.64 = 3.36T = 0.75 * WBD = 2.52 deg.C• Final T = 35 – 2.52 = 32.48 deg.C

WBD

Answer to problem 8• (b). At initial conditions,

– Tdb = 35 deg.C– RH = 25 %

• Answer– the specific volume is 0.8853 m3/kg, humidity ratio is 0.

0088 kg/kg of dry air. – At wet bulb of 20.15 and dry bulb of 23.87, the humidit

y ratio is 0.0132 kg/kg of dry air. – A cubic meter of air will contain 1 m3/0.8853 m3/kg = 1.

13 kg. – The change of humidity ratio is 0.0132 – 0.0088 = 0.0

044 kg/kg dry air.– Thus, the total water gained by each cubic meter of o

utdoor air will be 1.13 kg * 0.0044 kg/kg = 0.005 kg.

A small bug was found in the software. Both Twb should be the same.

Psychrometric Process

9. A ventilation system for a calf nursery draws 1 m3/s of air from the nursery, exhausts 0.2 m3/s of the air to the outside, and replaces the exhausted air with fresh air. Air in the nursery is at 20 deg.C and 50 % relative humidity, and outdoor air is at 5 deg.C and 80 % relative humidity. Determine properties of the mixed air when it is returned to the nursery.

Calf Nursery

20 deg.C, 50 % relative humidityMixing

chamber

Outdoors

5 deg.C

80 % relative humidity

0.2 m3/s

1 m3/s

#1 #2 #3

T, deg.C 5 20 16.86 (cal.)

RH, % 80 50 53.37 (psychart)

V, m3/s 0.2 0.8 0.999

, m3/kg 0.7935 0.8402 0.83

M, kg/s 0.252 0.952 1.204

W, kg/kg 0.0043 0.0073 0.0066 (cal.) / 0.0064 (psychart)

h, kJ/kg 15.859 38.545 33.796 (cal.) / 33.054 (psychart)

W3 = (M1*W1+M2*W2)/(M1+M2)h3 = (M1*h1+M2*h2)/(M1+M2)

T3 = (M1*T1+M2*T2)/(M1+M2)

State 3

2121

2

lengthMM

M

2121

1

lengthMM

M

Psychrometric Process10. You are designing an evaporative cooling system to ve

ntilate a controlled environment, totally enclosed, poultry house in a hot and dry climate.

Summer design weather conditions for your location are 40 degree C Tdb and 20 % RH.

75 m3/s of outdoor air will be drawn by the evaporative cooling system (75% efficiency) and 15 m3/s of outdoor air will be drawn into the house through infiltration.

The birds in the house are expected to produce 250 kW of total heat of which 40% is latent heat. Determine

a) the psychrometric properties of the air mixed by air through evaporative cooling pad and air through infiltration.

b) the psychrometric properties of the exhausted air. c) the volumetric flow rate of air exhausted from the house. d) the rate (L/s) at which water must be supplied to the evaporative c

ooling pads by a water pump. When a (water flow/evaporation rate) ratio of 3 is desire.

75 m3/s4

3

1

2

15 m3/s

Evaporative cooler

infiltration

Poultry House

State 1

State 2

State 3

= 16.66, = 86.3, = 102.96 kg/s

T3 = ( * T1 + * T2) / = 28.69 deg. C

W3 =( * W1 + * W2) / = 0.0137 kg/kg

Zoom out

1M

1M

1M

2M

2M

2M

3M

3M

3M

= 75 m3/s

2 = 0.8689 m3/kg

= 86.3 kg/s

431

2

= 15 m3/s

1 = 0. 9003 m3/kg

= 16.66 kg/s

Evaporative cooler

infiltration Poultry House

T1 = 40 deg. C

RH1 = 20 %

W1 = 0.0092 kg/kg

T2 = 26.51 deg. C

RH2 = 67.1 %

W2 = 0.0146 kg/kg

T3 = 28.69 deg. C

W3 = 0.0137 kg/kg

= 102.96 kg/s

100 kW latent

150 kW sensible heat

T = 150 / ( * Cp) = 150 / (102.96*1.006) = 1.448 deg. C

hfg = 2501 – 2.42 * (28.69+1.448) = 2428 kJ/kg

W= 100 / ( * hfg) = 100 / (102.96 * 2428) = 0.0004 kg/kg

T4 = T3 + T = 28.69 + 1.448 = 30.138 deg. C

W4 = W3 + W = 0.0137 + 0.0004 = 0.0141 kg/kg

3M

1M

2M

M

M

1V

2V

Zoom in to the limit.

State 3

State 4

smMV

MM/ 47.908787.0*96.102* 3

444

34

The volumetric flow rateand properties of states 3 and 4

the rate (L/s) at which water must be supplied to the cooling pad = 3 * (0.0146 – 0.0092) * 75 / 0.9003 = 1.3495 kg /s

Reference

Albright, L.D. 1990. Chapter 2 – Psychrometrics. Environment Control for Animals and Plants. An ASAE Textbook. ASAE, St. Joseph, MI, USA.